I need some expert advice on this valve. I think my wet tank drain may be plugged by junk. It turns freely at the large slot inside the screwed in fitting but does not get any tighter or looser.

Neither of my manuals shows anything about this drain except how to make the special tool to turn it?

It is some kind of spring thing because it won't screw out and I can push it up about 1/8" against a spring force.

I'm afraid to try to force anything up through the center drain hole. I tried wire while it had air pressure but nothing comes out. I know it has to have loads of junk in it, not even air comes out!! I need to find out how it is made.

I tried to screw out the whole large pipe thread fitting which contains the drain valve but I'm afraid that thing is in there forever!

Could it possibly be that you've got a cable pull drain and the cable is gone? If the last fitting is brass it should be easy to remove (I'd certainly do it when the system is at "0 PSI") then by using a compressor you could add just enough air to the system to clear any obstructions in the drain.

I'm not familiar with the GM's since My MC5a is my first bus, but my wet tank has a 90 Deg. fitting after the drain valve which is open when it appears to be closed and vice-versa.

Gus,Could it be that your wet tank is that full of crud, some of which has coagulated on the valve?I believe I'd get that fitting out and make sure the tank is clean and empty. There's no telling when the last owner drained it or how often. Or even if the compressor went bad at some time.If that tank is full, check the next tank in line for crud too.

The drain valve should come out. I removed ours a couple of years ago. Clean around the fitting so you can make sure nobody has spot welded the thing to the tank!

Get a 3/4" drive socket that will fit the "outer nut" and pull. Remember, that thing has been in there for years.

After I got the old one out, I sprayed degreaser solvent up into the tank to help break loose the gunk. I then replaced the old valve it with a new ball valve. All you'll need is a "close nipple" and the valve. Then when you get ready to drain the tank, you can take your foot and crack it open (keeps you off your hands and knees). This setup has worked for me.

If you haven't already done so, you might check the tank under the front axle and make sure it's OK also.

his valve is countersunk inside what I would call a pipe thread bushing made of brass (Thankfully). This bushing has about a 1 1/2" OD and is screwed up into the wet tank. There is a large slot, about 1/4" wide across the inside of the bushing that turns, supposedly to drain the tank.

I tried turning it with a medium size pipe wrench but do have a 3/4 " socket set which I will try next. Truthfully, I'm afraid of breaking it off or twisting the bottom out of the tank given that it is probably pretty rusty.

Are any GMC guys out there who know just how this thing is constructed?

For some reason this thing didn't jump to the front, give me some help here!!

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Hi Gus. I didn't jump on this, because I am not familiar with GMCs. However, when I first saw this thread I wondered if what you are looking at is the pressure relief valve. Today they are not adjustable, but many years ago they were.

I have seen some pretty old air tanks and every one I have seen has a simple ball valve or a chain pull drain valve. Nothing requiring a tool to operate.

If yours does not have one, you can simply remove a plug that is in the bottom part of your wet tank and screw in a ball valve.

Would it be possible to put a picture of the valve you are talking about on this thread so we can all see?

No matter what, you need an easy to operate valve in the bottom of the wet tank so that you can drain that tank often. Draining the tank is important to assure that it has sufficient air volume (volume not taken up by water) and to see if you are getting excess oil in the system (compressor going bad).

This is a drain valve and the making of a tool to open it is carefully explained in the 4104 Maint. Man. but it is not illustrated in the Parts Book or otherwise mentioned in the Maint. Man.

The Driver's Handbook mentions the importance of daily draining.

It sure would be nice to know how it is constructed.

I collect antique trucks and I have never seen an air tank mounted vertically like this and have never seen this type of drain valve. I can't even think of a good reason for using this type of valve.

My plan is to clean up the threads where it meets the tank and take it our one way or another. A big impact wrench is my first idea, heat after that and cutting out as a last resort. Since it is brass it should eventually come out.

I fully expect a deluge of crap to come gushing out the 1 1/2" hole. Once out I'll replace it with a bushing and a ball drain valve to eliminate this problem once and for all.

This tank is mounted vertically behind the left rear wheel with the drain valve at the bottom end and easy to reach, unlike the buried front tank.

My guess is this tank/valve setup is also used on the 4106 and probably all other GMCs of the mid fifties-sixties.

Gus, your drain valve could best be described as a fairly simple ball valve, all brass. Consists of an outer shell and what seems to be a tapered inner portion held in place by a spring and cover on the inside end held together by a keeper. I have not taken mine apart but it seems fairly simple. I replaced the one on my 4106 with a laniard valve. I'll attach some pictures for you. I see I am llimited on size of attachments, so here is the first one

Great pictures! I was beginning to think it was an automatic drain valve, but it is not. Hit the tank surrounding the valve with a propane torch for a couple of minutes (expansion of the two parts should help break the bond). Don't overdo the heat as some tanks have a corrosion resistant lining. Then hit it with a 6 point socket and impact wrench. It should come right out.